On the surface topography of ultrashort laser pulse treated steel ...

Applied Surface Science 258 (2011) 1555–1560Contents lists available at SciVerse ScienceDirectApplied Surface Sciencej our nal ho me p age: www.elsevier.com/loc ate/apsuscOn the surface topography of ultrashort laser pulse treated steel surfacesJ. Vincenc Obona a,1 ,V. Ocelík a , J.Z.P. Skolski b,c , V.S. Mitko b,c , G.R.B.E. Römer c ,A.J. Huis in’t Veld c,d , J.Th.M. De Hosson a,∗a Materials innovation institute M2i, Department of Applied Physics, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlandsb Materials innovation institute M2i, Mekelweg 2, Delft, The Netherlandsc University of Twente, Faculty of Engineering Technology and Materials innovation institute, P.O. Box 217, 7500 AE Enschede, The Netherlandsd TNO Science & Industry, Department Materials Technology, De Rondom 1, 5600 HE Eindhoven, The Netherlandsa r t i c l e i n f oArticle history:Received 19 April 2011Received in revised form28 September 2011Accepted 28 September 2011Available online 6 October 2011Keywords:Ultrashort laser pulsesRipplesBubblesJetsScanning electron microscopyUltra fast laser nano-machininga b s t r a c tThis paper concentrates on observations of the surface topography by scanning electron microscopy(SEM) on alloyed and stainless steels samples treated by ultrashort laser pulses with duration of 210 fsand 6.7 ps. Globular-like and jet-like objects were found depending on the various levels of the fluenceapplied. It is shown that these features appear due to solid–liquid and liquid–gas transitions withinsurface layer irradiated by intense laser light. The observations are confronted to the theory of shortpulsedlaser light–matter interactions, including interference, excitation of electrons, electron–phononcoupling as well as subsequent ablation. It is shown that the orientation of small ripples does not alwaysdepend on the direction of the polarization of laser light.© 2011 Elsevier B.V. All rights reserved.1. IntroductionFundamental understanding about the appearance of periodicsurface modifications due to the interaction of ultrashort-pulsedlaser light with materials is still incomplete although the topic hasbeen examined for almost half a century [1]. Several theoreticalmodels supported by experiments on metals were put forward todescribe the periodicity of surface characteristics [2–4] as well asthermodynamics of the irradiated material response to absorbedenergy [5–7]. To complete a list of theoretical models, the selforganizationmodel of Reif et al. has to be mentioned [8]. Thismodel explains the origin of surface corrugations on semiconductorsand dielectrics. The advent of ultrashort laser pulses also havegiven an impetus to industrial applications based on the creation ofsub-micrometer laser induced periodic surface structures (LIPSS, ingeneral ripples) aimed at achieving so-called lotus effect [9], specialoptical properties [10], precise micro/nano patterning, etc. [10].∗ Corresponding author at: Materials innovation institute M2i, Department ofApplied Physics, University of Groningen, Nijenborgh 4, 9747 AG Groningen, TheNetherlands. Tel.: +31 0 50 363 4897.E-mail address: J.T.M.De.Hosson@rug.nl (J.Th.M. De Hosson).1 On leave from Institute of Electrical Engineering, Slovak Academy of Sciences,Dúbravská Cesta 9, 841 01 Bratislava, Slovakia.The former theory of the ripples formation was based on modulationof energy input into a sample surface caused by interferenceof “scattered” fields with the refracted laser beam. The “scattered”fields are due to the surface roughness, which is confined in regionreferred to as the selvedge region [11]. This theory based on interference,known as the Efficacy Factor Theory, has an ability toexplain the surface features observed after laser–matter interactionvia a purely electromagnetic approach considering a changeof refractive index of a treated material [12]. According to thisapproach, the LIPSS with different texture and periodicity mayappear on treated surfaces depending on the direction of polarizationand wavelength. Ripples are either perpendicular or parallelto the polarization vector but they do not strictly follow theseorientations due to micro-level geometry and/or chemical inhomogeneitiesat surfaces. For ps and fs pulses the periodicity of lowspatial frequency LIPSS (LSFL, or big ripples) may be significantlysmaller than the laser light wavelength. The Efficacy Factor Theorybased on “scattered fields” interference predicted the decreaseof the LIPSS periodicity and high spatial frequency LIPSS (HSFL, orsmall ripples) parallel to polarization but it is weak in predictionsof the periodicity, height and width on pulse-to-pulse basis. Influenceof laser fluence on LIPSS growth is open question as well [12].Therefore, the classical approach has been revised [13]. This revisedmodel deals with the LIPSS creation via initial surface electromagneticwave (surface plasmon) interference with the laser light and0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2011.09.130

1556 J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–1560subsequent grating-assisted surface plasmon–laser coupling. It canprovide a view of LSFL growth [13] on various materials includingmetals, semiconductors and dielectrics for multiple exposures.However, it is still unable to give a full explanation of HSFL creation.In this work, we show that surface features created by laserpulses on alloyed and stainless steel start at low fluence with smallsphere-like objects (bubbles) followed with increasing fluence byelongated HSFL parallel to laser beam polarization vector. At evenhigher fluence the coexistence of orthogonal system of HSFL andLSFL, was observed. Surprisingly, as shown in this paper, HSFLlooses its polarization vector dependency in the presence of LSFLat high fluences. The HSFL becomes perpendicular to sidewalls ofthe LSFL. For explanation of this phenomenon, effects beyond thepurely electromagnetic have to be taken into account. Observationspublished in ref. [6] suggest that also relaxation of the absorbedenergy plays a significant role in the surface objects creation.The objective of this work is to compare two surfaces of steelwith slightly different chemical compositions. Two pulse durations(fs and ps), were used in order to compare influence ofpulse duration and electron–phonon relaxation time on the surfacetopography. Detailed inspections by SEM have been made. Crosssectionsof the surface objects have been prepared by focused ionbeam technique to allow measurement of spatial dimensions of thesurface features.2. ExperimentalIn the experiments two different materials as well as laser pulsegenerating systems were used.A titanium sapphire based laser system (Coherent RegA) witha wavelength of 800 nm was applied for machining of 800H hightemperature alloy. The laser system generates of 210 fs laser pulseswith a Gaussian distribution. The system delivers the pulses at afrequency of 50 kHz. Average powers of 5, 10, 25 and 30 mW wereapplied. The experiment was performed as an exposure in sets ofparallel lines under various conditions. Laser beam scanning speedwas set to 50, 100, 200, 400 and 800 mm/s. The effect of multipleenergy delivery to the same surface area was realized by applying2, 5, 10 and 20 overscans. Diameter of the laser beam was 20 m.800H steel was used as a substrate. Its chemical composition islisted in Table 1. The surface was chemically etched prior to laserprocessing to highlight grain boundaries on the surface [14].An ytterbium-doped YAG (Yb:YAG) system (Triumph TruMicro)with a central wavelength of 1030 nm was used for machining ofstainless AISI 304L steel. Its composition is presented in Table 1. Thelaser system generates laser pulses (repetition rate 50 kHz) witha maximum of 500 mW average power. We used 50 mW powerfor the samples treatment. The duration of Gaussian shaped laserpulse was 6.7 ps in all experiments. A combination of a rotary /2wave plate and a beam splitting cube served as a power attenuator.The laser light was linearly polarized. Manipulation of the beamover the sample was accomplished by a two-mirror galvo-scannersystem (Intelliscan 14 of ScanLab, Germany). A 100 mm telecentricf-Theta lens (Ronar of Linos, Germany) for 1030 nm wavelengthfocused the beam to a circular spot with diameter of 28 m. Theaverage power was measured at the exit of the scanner system by apower meter. In all experiments the normal incidence of laser lightwas used. The processing conditions used in the experiments arelisted in Table 2.Two different microscopy techniques were used to investigatethe sample surfaces treated by ultrashort laser pulses. A PhilipsXL30 SEM equipped with a field emission gun offers a lateral resolutionof the surface objects at a level of few nanometers. The lackof height information of the objects was partially compensated byobservations on tilted surfaces. The exact profile of treated surfaceFig. 1. SEM micrograph of the 800H steel sample treated by a single 210 fs laser pulsewith the 0.1 J pulse energy. The sample is tilted vertically in 55 ◦ from normal view.The centre of application of laser beam is located at the centre of the micrograph.RI, B and HSFL area denote random indents, bubbles and area covered by HSFL,respectively.was observed by SEM after cross-sectioning it using focused ionbeam (Tescan Lyra FIB-Field Emission Gun) with Pt depositedprotection.3. ResultsScanning electron microscopy observations of treated surfaceswere performed on both materials. In order to avoid confusionwhen using the term fluence for overlapped and overscanned laserpulses in the following, we mention rather the delivered energy asa combination of pulse energy, overlap (Eq. (1)) and the numberof overscans instead of overall, accumulated or absorbed fluence.Overlap is defined as:O = D − pD × 100% (1)where D is focus spot diameter and p is the pitch (pulse to pulsedistance) expressed as scanning speed v, divided by the repetitionrate f:p = v fIn the case of 800H steel treated by 210 fs pulses, the lowestenergy delivered on the surface was reached by setting of0.1 J pulse energy and 20% overlap without subsequent overscans.Detailed inspection of Fig. 1 reveals predominance of randomindents (RI) at the margin of the beam and many bubble-like objects(B) within the whole observed area. At the highest energy (centreof Fig. 1), the protrusions started to be organized into aligned linearelongated objects (HSFL).The energy delivered in the experiments was increased in threedifferent ways: (i) by increasing of laser pulse energy, (ii) bydecreasing of laser beam scanning speed at fixed frequency leadingto an increase of overlap of subsequent laser pulses and (iii) by multiplescanning over the same laser tracks (2, 5, 10 and 20 overscans).The laser track presented by SEM pictures in Fig. 2 was obtained byscanning of the 800H sample surface at 0.6 J pulse energy and90% overlap. Fig. 2a shows the end of such laser track. The evolutionof the surface objects as a function of the amount of energycan be observed in the middle of this micrograph from the rightto the left side. Appearance of the surface is continuously changingfrom (i) slightly modified surface at the right laser track marginfollowed by (ii) poorly aligned HSFL (vertically oriented), (iii) wellaligned HSFL and (iv) well aligned HSFL on poorly developed LSFLat the area of the steepest increase of the delivered energy. Finally,the surface gets also (v) well-defined LSFL (horizontally oriented)covered by discontinuous HSFL in area with highest energy input.Close-up of Fig. 2a displayed on Fig. 2b reveals the transition from(2)

J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–1560 1557Table 1Chemical composition of 800H and AISI 304L stainless steel substrates.C (wt.%) Cr (wt.%) Fe (wt.%) Ni (wt.%) Mn (wt.%) Si (wt.%) P (wt.%) S (wt.%) Al (wt.%) Ti (wt.%) Al/Ti (wt.%)AISI 304L 0.03 18.0–20.0 Bulk 8.0–12.0 2.0 1.0 0.045 0.03 – – –800H 0.06–0.1 19.0–23.0 Bulk 30.0–35.0 – – – – 0.15–0.6 0.15–0.6 0.85–1.2Table 2Summary of the processing conditions: from left to right: – laser light wavelength, – pulse duration, ss – spot size, f – repetition rate, P – average laser power, E p – pulseenergy, – incidence angle, v – scanning speed, O – overlap and OS – number of overscans. (nm) (fs) ss (m) f (kHz) P (mW) E p (J) ( ◦ ) v (mm/s) O (%) OS800H 800 210 20 50 5–30 0.1–0.6 90 50–800 20–95 2–20304L 1030 6700 28 50 50 10 90 50, 1000 29, 96 5, 50region covered by HSFL (the right side) to the area of coexistenceof HSFL and LSFL together. The rounded-ridge shape of the HSFL aswell as plenty of globular objects on the surface (Fig. 2b) suggestthe formation of the HSFL from the liquid state when using 210 fslaser pulses.The orientation and spacing of HSFL and LSFL were quantifiedusing 2D Fourier transform of SEM images (not shown here).Spacing and height of the surface features were investigated oncross-sections prepared with the aid of FIB technique. Spacing ofHSFL in the areas with low energy delivered is ranging over anaverage distance ∼200–400 nm, and height ∼40–60 nm (Fig. 3a).The LSFL showed an average periodicity of ∼650 nm and height of200–300 nm (Fig. 3b).The final increase of delivered energy in this experiment wasreached by making an overscan of already existing laser tracks.Fig. 4 shows a wavy surface of the same 800H steel sample treatedwith laser pulse energy 0.6 J, 80% overlap and 10 overscans. Herethe LSFLs are oriented vertically (laser track follows up-downdirection on the micrograph, polarization vector is horizontal) andHSFL are again perpendicular to these. The spacing of LSFL is comparableto the one in Figs. 2 and 3b (∼670 nm). The difference tothe previous energy level (Fig. 2) lies in a stronger development ofLSFL with deeper valleys between them. The HSFL bridges the LSFL.Again, the globular objects are present at the surface and togetherwith round-ridge shapes of HSFL (like frozen liquid) suggest thatthe formation of the liquid state is the last step in process.Another experiment with wavelength of 1030 nm and pulseduration of 6.7 ps (see Section 2) shows at high delivered energy(1 J pulse energy, 96% overlap and 5 overscans) a slightly differentpicture of HSFL and LSFL (Fig. 5). In this case, LSFL (orientedperpendicular to polarization vector) are not continuous butinterrupted quite frequently. The direction of HSFL is not anymorecontrolled by the orientation of the polarization vector butthey are always located inside valleys between the LSFL andoriented perpendicularly to their sidewalls. Therefore, in someplaces where LSFL are interrupted one may observe HSFL withthe direction even perpendicular to the vector of polarization.This observation is the clear evidence that a formation of HSFLstrongly depends on the roughness of treated surface. When LSFLare already present, the direction of HSFL is not controlled bypolarization direction anymore. The observations suggest that aprocess related to melting of the material caused by laser pulseFig. 2. SEM micrograph of the 800H steel sample treated by 210 fs laser pulses with 90% overlap and pulse energy 0.6 J. (a) Areas with different delivered energies aremarked at the end of laser track. (b) Detail from the area with highest delivered energy shows coexistence of small and big ripples. Sample was tilted vertically 55 ◦ from thenormal view.Fig. 3. SEM picture of (a) HSFL and (b) LSFL on cross-sections produced by FIB. Process parameters of the laser track preparation are: (a) 0.6 J pulse energy, 20% overlapand one scan; (b) 0.6 J, 96% overlap and 20 overscans. Images are tilted 55 ◦ in vertical direction. Pt-IBID and Pt-EBID are protection Pt layers applied by so-called ion beaminduced deposition and electron beam induced deposition, respectively.

1558 J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–15604. DiscussionFig. 4. SEM micrograph of the 800H steel sample treated by 210 fs laser pulses, 0.6 Jlaser pulse energy, 80% overlap and 10 overscans. Periodicity of LSFL is indicated.Sample tilted vertically 55 ◦ from normal view.Fig. 5. SEM micrograph of the 304L steel sample treated by 6.7 ps laser pulses, 1 Jpulse energy, 96% overlap and 5 overscans. LSFLs have horizontal direction (perpendicularto polarization). Orientation of small ripples is perpendicular to the sidewallsof LSFLs.impingement is most responsible for shape and direction of theHSFL.Although Figs. 2b, 4 and 5 already indicated traces of liquidphase, Fig. 6 shows these features in a more pronounced way. Manydroplets appear on the surface as well as the small resolidifiedjet-like objects (marked by arrows) with spherical top ends.Fig. 6. SEM micrograph of the 304L steel sample treated by 6.7 ps laser pulses, 1 Jpulse energy and 29% overlap. The arrows indicate resolidified droplets and smalljet-like objects with spherical ends. Inset of the micrograph shows details of suchfeatures captured at 35 ◦ vertical tilt from normal view.In literature it has been shown that even for laser pulses withduration shorter than few picoseconds, solid (s)–liquid (l)–vapor (g)transitions may play significant role in the process of the absorbedenergy relaxation, on irradiated metal surfaces [15]. In the work ofKorte et al. [16], the influence of electron–phonon coupling (EPC)was discussed in order to explain the different nature of materialablation for transition Cr and noble Au metals. They mentionedweak EPC of Au to be responsible for strong melt dynamics andsubsequent ablation to occur out of the molten phase. In the caseof Cr, the strong EPC should cause ablation of the material almostwithout melting.Two kinds of high-alloyed stainless steels 800H (Cr 19–23 wt.%,Ni 30–35 wt.%) and AISI 304L (Cr 18–20 wt.%, Ni 8–12 wt.%) widelyused in industrial applications are investigated in this work. Incontradiction to the observations in ref. [16], we observed on oursamples a significant appearance of liquid phase in the form of individualspheres and rounded spherical knops on the top of laterallyelongated features, at very low applied energy. With increasingenergy, the liquid phase features lie on the surface in the form ofHSFL, crossing the LSFL, together with substantial amount of sphericalobjects on the surface. Finally, for the highest applied energythese liquid-like objects in the form of jets with globular ends fillup the grooves between LSFL, again with a substantial amount ofisolated spherical droplets on the surface.In order to explain the appearance of the resolidified surfaceobjects, we will divide the laser–matter interaction into these threesubsequent steps, according to the time scales and the nature of theprocesses involved:• Modulation of laser light absorption described by Efficacy FactorTheory and/or initial surface plasmon interference with thelaser light followed by grating-assisted surface plasmon–lasercoupling [11–13], and laser energy absorption by free electrongas.• Transfer of the absorbed energy to the phonons described bywidely accepted two-temperature model [17].• Response of the irradiated material to the energy transfer [6,7].4.1. Efficacy Factor Theory, surface plasmon interference followedby grating-assisted coupling – energy absorptionLaser pulse duration, wavelength, beam incidence angle, samplesurface roughness and refraction index of the irradiated materialare the process parameters that strongly govern the laser energydeposition in these theoretical approaches. Both models are ableto explain the origin of periodic modification in experiments butfail in particular cases. The objective of this paper is not to evaluatein detail which of the approaches is more appropriate buthere we concentrate on an explanation of the orientation of theHSFL with respect to polarization that was found either parallel ofindependent at higher applied fluences.As pointed out in ref. [12], it is difficult to predict the LIPSS characteristicson a pulse-to-pulse basis via the Efficacy Factor Theorydue to (i) evolution of surface roughness and simultaneously (ii)poor knowledge of changes in the refraction index during the laserpulses. In (i), the surface roughness has a stochastic nature and ischaracterized by spherically shaped islands. This assumption hascertain restrictions for HSFL and LSFL already developed after thefirst couple of pulses. However, the periodicity can be predictedfor lower roughness. Also the orientation parallel to polarizationfor deep-sub-wavelength, so-called dissident (or type-d) structurescan be described [12]. For (ii), the lack of knowledge of changesin the refraction index was circumvented by the introduction ofan effective refraction index. These computational results showed

J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–1560 1559that the approach has an ability to model periodicities for LSFLsmaller than the initial laser wavelength and HSFL fringes parallelto polarization, as well. However, quantitative predictions ofLIPSS periodicity, width and height on a pulse-to-pulse basis arenot feasible yet [12].Huang et al. [13] used another model with excellent ability to fitexperimental results on various materials and to read out dielectricfunction of the treated material. The model deals with a modulationof absorbed energy via the interaction of highly excited surfacestates, surface plasmons, with incoming laser light. For multiplepulses, so-called grating-assisted coupling is responsible for deepeningof the surface ripples and shrinkage of the ripples periodicity.In fact, although the model seems to be very successful in multipulseexposure description, it apparently fails in the descriptionof few pulses experiment. The model assumes that the surfaceplasmon–laser interference is responsible for the initialization ofLSFL and the transverse magnetic characteristic of surface plasmondetermines the polarization dependence of the ripples [13]. It isclear that the model definitely can not describe the creation of HSFLparallel to polarization and nothing can be said about polarizationindependent structures at higher fluencies.In general, these two models predict where the absorption couldbe expected on the sample surface. The laser energy is absorbedby free electron gas excitations. In a metal they exclusively occurvia inverse Bremsstrahlung and consecutive collisions among electronslead to the thermalization within the electron gas on a timescale of about 100 fs [18].4.2. Transfer of absorbed energy to the phononsIn the previous section, two models of modulated energyabsorption are used to describe where the delivered energy isabsorbed. However, they do not predict what the response of theirradiated material to this absorbed energy will be.In the following step, delayed up to tens of picoseconds, thetransfer of energy from overheated electron gas to the phononsoccurs. Evolution of the electron gas and the lattice temperatures isdescribed by two-temperature model [17]. Two of the most importantparameters in the model are electron thermal conductivity K eand so-called EPC constant G. The higher the electron conductivitythe lower is the peak electron temperature on the surface due to amore efficient electron thermal diffusion into the bulk material (inthe case of noble metals). On the other hand, a higher values of G andlow values of K e (most of the transition metals and in the case of Ni,Fe, Cr based steels) lead to a faster equilibrating dynamics betweenthe lattice and the electron baths towards the common equilibriumtemperature. Comparison of the electron thermal conductivitiesand electron–phonon coupling constants for two typical representatives(Ni, Au) of the metal groups were chosen from [19]. TheK e,Ni /K e,Au ≈ 0.3 and G Ni /G Au ≈ 18 ratios, respectively, clearly show,that for the same applied energies, much faster (violent) transferof the much less diffused energy occurs, in the case of low K e andhigh G metals. It means that for Ni and metals with similar materialconstants much higher surface temperatures should be expectedin comparison to noble metals.4.3. Response of the irradiated materialSo far, we mentioned that it is possible to explain the occurrenceof preferentially absorbed energy according the Efficacy Factor Theoryand surface-plasmon-interference-model. Here, we will answerthe following questions:(i) What is the nature of the HSFL growth at low fluences (fewapplied pulses)?Fig. 7. Schematic drawing of laser–matter interaction shows our hypothesis forlaser light absorption: (a) for low energy and smooth surface from top view as well as(b) for high energy and rough surface. HSFL and LSFL mean absorption of the energyalong HSFL and LSFL directions, respectively. Drawing (c) represents the side viewof expansion of high energy irradiated material with part of the material subjectedto violent solid–gas transition (s–g) in his upper layer and part to melting (s–l) orphase explosions (l–g) in deeper layer.(ii) How can we explain the transfer from HSFL to LSFL?(iii) What is the nature of the HSFL growth between the LSFL athigher fluences?Current analytical methods do not allow a direct investigationof the HSFL and LSFL formation by spatial-resolved methods withnanometer resolutions. Moreover, the Efficacy Factor Theory hasnot provided a suitable tool to calculate the laser light absorptionon pulse-to-pulse basis, up to now. Restriction of surface-plasmoninterference-modelin description of HSFL parallel to polarizationwas mentioned, as well. To describe fully both kinds of HSFLs formation,we should consider these four factors: absorption, electronthermal conductivity, electron–phonon coupling and subsequentrelaxation of the irradiated volume.Exposure of the sample surface with low pulse energies at highprocessing speed shown in Fig. 1 can be considered as a single pulseimpingement. Marginal part of the spot reveals random depositionof energy, while in the centre, at slightly higher deposited energy,first HSFL collinear to polarization vector appeared. These can belinked to HSFL type-d structures in ref. [12]. However, in Fig. 2a(right side of the picture), we can clearly see the HSFL type-d structureseven for multi-pulse exposures (90% overlapped laser spots)on rougher surface due to previous pulse impingements. Here, thelaser light absorption is still collinear to polarization (see Fig. 7a),even thought the delivered energy is higher in comparison to Fig. 1.On the other hand, in the case of high delivered energy(Figs. 5 and 6) on rough surfaces there is a possibility of higherenergy absorption (Fig. 7b). We observe the presence of the welldeveloped LSFL (type-s structures in ref. [12]) orthogonal to thepolarization vector with HSFL almost all lying between sidewallsof the LSFL. It seems that the side walls takeover control of theHSFL direction because the HSFL completely lost dependence onthe polarization vector (Fig. 5). Moreover, many jet-like objects are

1560 J. Vincenc Obona et al. / Applied Surface Science 258 (2011) 1555–1560present with spherical endings (Fig. 6 – white arrows). It suggeststhat the growth of HSFL has a different origin in this case. Nevertheless,both HSFL at low energies and HSFL between LSFL, areassociated with a liquid phase formation.Based on Lambert–Beer law of radiation absorption in a materialthere is a different portion of energy absorbed by electrongas excitations at different depths under the surface. This energyis delivered to phonons. Molecular dynamics modeling by Lewisand Perez [7] showed different nature of the absorbed energyrelaxations for two energy levels equal to 1.2× and 2.8× ablationthreshold energy, respectively. The single-pulse simulationsrevealed for 1.2 multiple of ablation threshold energy only tworegions, non-ablated and porous (voids filled by gas). However, for2.8 multiple of ablation threshold energy they refer to the formationof much more intricate profile including spallation, phase explosion,fragmentation and vaporization, crossing the profile from bulkto the surface side. It is worth mentioning that pathways of isochoricexpansion in temperature–density diagrams [7], the lowerparts of the irradiated volume follow solid–liquid–vapor transitionsfor both energy levels.In summary, taking into account the particular aspects oflaser–matter interaction the answer to ‘What is the nature of theHSFL growth at low fluences (few applied pulses)?’, is as follows. Inthe case of a slightly higher energy than the ablation threshold andan initially smooth surface, the energy absorption on steel surfacesoccurs preferentially parallel to polarization vector (Fig. 7a). Theobservation was supported by computational results originated inEfficacy Factor Theory, [12]). Response of the irradiated volume tosuch a low energy on the absorption places are gentle (s)–(l)–(g)transitions, which cause slight melting of the surface (Fig. 3a).For answering the second question, ‘How the transfer from HSFLto LSFL could be explained?’, multiple-laser-pulses energy absorptionon already roughened surface (HSFL) has to be taken intoaccount. In this case, the Efficacy Factor Theory is still applicablefor the initial few pulses. However, only when the amplitude ofthe surface wavy ripples is much smaller than the wavelength ofthe laser light a mutually orthogonal system of ripples is visible.If the ripples amplitude becomes comparable to the wavelength,the ripples transversal to the polarization start dominate (Figs. 7band 2a,b). Here, the surface-plasmon-interference-model, whichpredicts the surface plasmon propagating only in direction transverseto the polarization [13], is more suitable for the descriptionof our observations. Moreover, our observations of LSFL periodicitysmaller than the wavelength are in accordance with prediction ofthe model presented in [13], as well. Previous discussion showedthat both models are needed in the description of the HSFL parallelto polarization and transform from HSFL to LSFL, as well.Finally, we address the third (What is the nature of the HSFLgrowth between the LSFL at higher fluences?) question. Accordingto the surface-plasmon-interference-model in combination withgrating-assisted coupling for multiple exposures, one could expectmuch higher energy absorption into progressively deeper ripplegrooves [13]. Following the energy transition from electron gas tophonons, an intricate profile of expanding material is developed(Fig. 3 in [7], Fig. 7c). In the lower parts of the expanding volume theabsorbed energy relaxes through a process called phase explosion(explosive boiling). The HSFL in this case is formed as common wallsbetween adjacent bursting bubbles. LSFL is then formed duringmultiple laser shots as hills between individual expanding events,see Fig. 7c. That is how we can explain independence of HSFL onpolarization and the direction orthogonal to sidewalls of the LSFL(see in Fig. 5).5. ConclusionsScanning electron microscopy and focused ion beam facilitieswere used to study the topography of 800H and AlSl 304L steelsurfaces after its treatment by 210 fs and 6.7 ps laser pulses withrepetition rate of 50 kHz and pulse energy 0.1, 0.2, 0.5, 0.6 and10 J, respectively. Based on current knowledge and our experimentalobservations we concluded that all three processes: (i)modulation of absorbed energy and the absorption of laser lightby free electron gas, (ii) diffusion of the thermalized electrons and(iii) electron–phonon coupling have to be considered as key factorsfor overall explanation of the LIPSS formation. The conclusions aredetailed as follows:Evolution of the surface objects created by laser pulses at lowpulse energy (∼ablation threshold) starts with the formation ofsmall spherical objects followed at increasing pulse energy withelongated HSFL oriented parallel to polarization vector of laserbeam. They have a characteristic height of 30–60 nm with a distancebetween them about 100–250 nm.With increasing energy a mixture of HSFL and LSFL, perpendicularto each other, appears. The LSFL are typically 100–400 nm inheight and the distance between them is equal or slightly shorterthan the wavelength of the laser light used.At high energy the direction of HSFL becomes independent onthe polarization. Simultaneously, HSFL lie at the bottom of the valleysbetween the LSFL.AcknowledgementsThis research was carried out under project numberM61.3.08300 in the framework of the Research Program ofthe Materials innovation institute M2i (www.m2i.nl).References[1] M. Birnbaum, J. Appl. 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